<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <script>
        var arr = [1, 2, 3, 4, 5, 6, 7, 8]
        /* [[1,2,3],[4,5,6],[7,8]] */
        /* 
        slice(0,3)
        slice(3,6)
        slice(6,9)
         */
        var result = [];
        for (let i = 0; i < arr.length; i += 3) {
            var item = arr.slice(i, i + 3)
            result.push(item);
        }
        console.log(result)
    </script>
</body>

</html>